The embodiments of the present invention relate to communication systems and, more particularly, to methods and corresponding systems for hybrid wired-wireless and wireless-wireless, point-to-multipoint communication, featuring a shared channel, discrete multi-tone modulation, and wireless transmission.
Basic principles and details relating to hybrid wired-wireless point-to-multipoint communication systems needed for properly understanding the embodiments of the present invention are provided herein. Complete theoretical descriptions, details, explanations, examples, and applications of these and related subjects and phenomena are readily available in standard references in the fields of digital telecommunication.
Known wireless modems take information from customer modems (Cable/CATV modem, xDSL modem or PON modem) and remodulate it in the air between the wire's endpoint and various wireless devices. These wireless technologies may vary and include technologies such as WiFi, WiMAX, BlueTooth, ZigBee and UWB.
Cable modems mostly use the DOCSIS standards for transferring data in parallel with dedicated CATV channels, which transfer the video channels over coax cables. Various modulations can be used to carry the data over the coax, while the most common modulation used today over Coax is single carrier. In xDSL modems a similar approach is used for carrying data over twisted pairs used by the PSTN infrastructure. The most common modulation used in xDSL is DMT/OFDM, even though single carrier QAM modulations are used as well in certain standards.
Some embodiments of the invention feature multi-carrier modulation. Multi-carrier modulation systems generally involve a data signal made of successive symbols, split into several lower rate signals, each associated with a sub-carrier and resulting in a long symbol time in comparison to the expected multipath delay spread. Orthogonal frequency division modulation (OFDM) is a multi-carrier modulation scheme, which maps data symbols onto N orthogonal sub-carriers, separated by a distance of 1/T, and where T is the useful symbol duration. In OFDM, cyclic guard intervals are frequently used to improve performance in the presence of a multipath channel. OFDM has become attractive for wireless communications due to its high spectral efficiency and resistance to noise and multipath effects. OFDM has been the foundation of a number of wireless broadcast standards, some of them providing for Single Frequency Network (SFN) operation, in which a number of transmitters operate in simulcast manner.
OFDMA is the “multi-user” version of OFDM. Each OFDMA user transmits symbols using subcarriers that remain orthogonal to those of other users.
The orthogonal frequency division multiple access (OFDMA) system, a multiple access system designed for simultaneous access by multiple users, is applied to OFDM. OFDMA divides an allocated frequency band into N subcarriers and allocates them to groups, for simultaneous use by multiple links. Supporting high rate applications, multiple subcarriers may be assigned to a single user. On the forward link from a base station to a plurality of users, the subcarrier groups, allocated to the respective mobile stations, are transferred simultaneously, while at the same time synchronizing with one another, and thereby guaranteeing mutual orthogonality of the subcarriers.